This invention is directed to a valve structure and, more particularly, to a novel flow passage between the seat port and outlet port of a valve having an axially movable closure member. More specifically, this invention is directed to a flow passage in a valve body having an axially movable closure member which provides a low fluid pressure drop across the valve when the closure member is moved a limited distance to its fully open position.
Although certain features of this invention may be equally applicable to conventional valves employing an axially movable closure element, this invention is particularly useful in the manufacture of valves which include a sealing element such as a metal diaphragm or bellows type seal between the valve flow passageway and the operating mechanism to protect against external fluid leakage through or around the operating mechanism. Such valves have found considerable utility as isolation or shut-off valves for toxic or otherwise dangerous fluids and for borated water safety systems in nuclear fueled power generating plants. These valves are normally used in the closed position to shut-off or isolate some fluid under pressure from a flow system and in the open position to deliver the pressurized fluid to the flow system. In the open position it is preferred that the pressure drop across the valve be minimized to ensure that the fluid is delivered to the flow system under sufficient pressure to satisfy the requirements of that system.
The fluid pressure drop across a valve is a function of fluid dynamics as applied to the geometry of the flow passageway provided through the valve body which is restricted to some extent by the mass of the body required to contain the fluid pressure and the provision of operating means and a closure member which, even when moved to the fully open position, normally provides some restriction in the cross-sectional area available for fluid flow through the valve. Efforts to reduce the pressure drop across a valve have traditionally taken one or both of two approaches -- namely, to open the flow area so the internal restriction will not produce fluid flow of a velocity high enough to dissipate an inordinate amount of fluid pressure in the form of heat energy, or in the case of symmetrical valves having a straight through port in the closure member, to provide a transition region of gradually increasing cross-sectional area in the outlet port of the valve body, see for example U.S. Pat. No. 3,643,914 which issued to E. A. Bake on Feb. 22, 1972. The former has been applied in the design of isolation valves having a disc or globe type closure member axially movable toward or away from a valve seat. That is, the axial distance the closure member is moved away from the seat to open the valve has traditionally been designed to provide a annular orifice of sufficient area to provide a volume of flow which will not dissipate an inordinate amount of pressure force in the form of heat energy. Conventional isolation valves employing axially movable disc or globe type closure members are designed to permit the closure member to move a distance on the order of from about one-fourth to about three-fourths the diameter of the seat port to provide a annular orifice of substantial area around the periphery of the closure member.
Unfortunately, the same degree of axial movement of the closure member relative to the seat port diameter is not economically feasible in a metal diaphragm or bellows sealed valve employing similar principles of operation. The maximum distance the closure element in a metal diaphragm valve can be moved is limited by the axial extent of deformation or flexure of the metal diaphragm. The extent of axial movement provided by a metal diaphragm will depend upon the dimensions and physical characteristics of the diaphragm but it will normally be on the order of about one-eighth the diameter of the seat port or less than about one-half the distance of axial movement preferred for a conventional isolation valve of the same size. The use of a metal diaphragm sealing element thus places a severe restraint on the distance the closure member may be moved from the valve seat to the fully open position and therefore normally increases the fluid pressure drop of the valve by restricting the volume of fluid flow between the closure member and the valve seat. A similar problem exists in the case of a bellows sealed valve where the maximum movement of the closure member is limited to that distance which is permitted by axial movement of a bellows seal of reasonable length.
Thus, the problem of pressure drop in fluid flowing through a metal diaphragm or bellows type isolation valve becomes more acute than that encountered in conventional isolation valve designs employing similar principles of operation by reason of the restricted or limited lift provided to the closure member through either of these sealing means. These and other valves having an axially movable closure member which is moved a maximum axial distance of about one-eighth the diameter of the seat port or less will be referred to hereinafter as "low-lift" valves.
With the foregoing considerations in mind it is a principal purpose and object of this invention to provide a valve having an axially movable closure member with improved pressure recovery in fluid passing therethrough by means of a novel flow passage between the seat port and the outlet port of the valve.
Another object is to provide a flow passage in an isolation valve having a low-lift axially movable closure member which reduces the pressure drop in fluid flowing through the valve in the fully open position.